Polymerase chain reaction (PCR) amplification of a portion of the genome of both rapidly growing Mycobacteria and Nocardia species, followed by restriction fragment length polymorphism (RFLP) analysis of the amplification products, has proven to be a useful technique in the diagnostic laboratory. Identification of these organisms at the species level can be obtained within a few days of organism isolation, as compared with the month or more required for conventional identification based on biochemical testing. In addition, these molecular procedures allow more accurate discrimination among species and subspecies than is possible with biochemical testing. Our work with two different areas of the Nocardia genome (a portion of the gene for 16S ribosomal RNA and a portion of the gene for the heat-shock protein) has suggested the existence of hitherto unrecognized Nocardia species; work is ongoing to characterize these organisms further. In addition, we have found some species of Nocardia to be human pathogens that were not previously reported to cause disease in patients in the western hemisphere, or were not known to be human pathogens at all, so far as we know. A manuscript describing our methodology has been published. We are currently writing up some of our findings on the unusual Nocardia species that we have found to be agents of disease in man.